The following article should help:
http://reviews.cnet.com/8301-33199_7-57603018-221/hdmi-2.0-what-you-need-to-know/
Basically it is a hardware change. Sony stated their current products will only need a BIOS upgrade to be fully compatible with HDMI 2.0. However, HDMI.org, the nonprofit group that oversees the development of the HDMI standard states that a simple BIOS update is not going to be a universal solution.
The article also states that current HDMI 1.4 cables are fully compliant with HDMI 2.0. Therefore, with the exception of Sony (for now) HDMI 2.0 will require a hardware upgrade.